The goal of this research project is to define the structure and radioactive label for ligands that will permit quantitative measurement of serotonin transporter (5-HTT) sites in living brain by external imaging with positron or single-photon emission tomography (PET or SPECT). The serotonin transporter is known to play a role in several physiological and pathophysiological processes, especially in depression, anxiety, and psychosis, as demonstrated by post-mortem and preliminary imaging studies. A radiotracer that bound specifically to 5-HTT would be of great utility in diagnosis, monitoring treatment, and research of such disorders. However, there is not yet a suitable tracer that allows quantitation in areas of the brain with low densities of 5-HTT, such as the cortex. In this application we propose to test the hypothesis that a novel structure, aza-bicyclo-[3.2.2]-nonane, will have the right combination of transporter binding affinity and pharmacological specificity to allow us to detect the serotonin transporter in the region of low density. In preliminary experiments using a classical structure-activity relationship (SAR) approach, we found that certain aza-bicyclo-[3.2.2]-nonane ethers showed nanomolar affinity to 5-HTT. We will extend this SAR to a larger library using parallel synthesis and purification. The ligand candidates will be screened for pharmacological affinity and specificity to 5-HTT versus competing binding sites. The most promising candidates will be radiolabeled with radionuclides, F-18, or 1-123 as appropriate to the molecule, and evaluated by imaging in vivo in non-human primates in a dedicated PET or SPECT imaging device.